1. Field of the Invention
This invention relates to a new process for the production of polyisocyanates containing uretdione and isocyanurate groups, i.e. polyisocyanate mixtures, by the oligomerization of a portion of the isocyanate groups of hexamethylene diisocyanate (hereinafter referred to as "HDI") using catalysts which accelerate both dimerization (uretdione formation) and also trimerization (isocyanurate formation), to the products obtained by this process and their use, optionally in blocked form, as the isocyanate component in polyurethane lacquers.
2. Description of the Prior Art
The production of polyisocyanate mixtures containing uretdione and isocyanurate groups by the dimerization and/or trimerization of a portion of the isocyanate groups of organic polyisocyanates using phosphorus-containing catalysts is known. These mixtures are known to be valuable starting materials for the production of polyurethane plastics. However, known processes for the production of such polyisocyanate mixtures using HDI as the starting material (cf. for example DE-OS 1,670,720 and DE-OS 3,432,081) are not optimally suited to large-scale production. The disadvantages of the known processes are the long reaction time and the relatively large quantities of catalyst which result in the need for a correspondingly large quantity of deactivators such that the end product contains a relatively high percentage of unwanted foreign components which adversely affect the properties of the polyurethane plastic.
In the process according to DE-OS 3,437,635, considerable quantities of alcohols are used as co-catalysts which means that valuable isocyanate groups are consumed, i.e. destroyed, by the addition reaction which takes place between isocyanate groups and hydroxyl groups.
Accordingly, an object of the present invention is to provide a new process for the production of HDI-based isocyanate mixtures containing uretdione and isocyanurate groups which is not attended by any of the disadvantages mentioned above.
According to the invention, this object is achieved by using HDI which is substantially free from carbon dioxide as the starting material. It is possible in this way to limit the reaction time to less than one working day, i.e., to less than 10 hours, and at the same time to carry out the process using minimal quantities of catalysts and without using large quantities of co-catalysts which consume isocyanate groups.